1. Technical Field
This invention relates to a light-emitting layer for use in inorganic EL devices, and more particularly, to a phosphor thin film used as a light-emitting layer and an EL panel using the same.
2. Background Art
In the recent years, active research works have been made on thin-film EL devices as small-size and large-size, lightweight flat displays. A monochromatic thin-film EL display using a phosphor thin film of manganese-doped zinc sulfide capable of emitting yellowish orange light has already become commercially practical as a dual insulated structure using thin-film insulating layers 2 and 4 as shown in FIG. 2. In FIG. 2, a predetermined pattern of lower electrodes 5 is formed on a substrate 1, and a first insulating layer 2 is formed on the lower electrodes 5. On the first insulating layer 2, a light-emitting layer 3 and a second insulating layer 4 are successively formed. On the second insulating layer 4, a predetermined pattern of upper electrodes 6 is formed so as to construct a matrix circuit with the lower electrodes 5.
Thin-film EL displays must display images in color in order that they find use as computer, TV and similar monitors. Thin-film EL displays using sulfide phosphor thin films are fully reliable and resistant to environment, but at present regarded unsuitable as color displays because EL phosphors required to emit light in the primaries of red, green and blue have poor characteristics. Engineers continued research on SrS:Ce (using SrS as a matrix material and Ce as a luminescent center) and ZnS:Tm as a candidate for the blue light-emitting phosphor, ZnS:Sm and CaS:Eu as a candidate for the red light-emitting phosphor, and ZnS:Tb and CaS:Ce as a candidate for the green light-emitting phosphor.
These phosphor thin films capable of emitting light in the primaries of red, green and blue suffer from problems of emission luminance, emission efficiency and color purity. Thus color EL panels have not reached the commercial stage. Referring to the blue color among others, a relatively high luminance is achieved using SrS:Ce. However, its luminance is still short as the blue color for full-color displays and its chromaticity is shifted toward green. There is a desire to have a better blue light-emitting layer.
To solve the above problem, thiogallate and thioaluminate base blue phosphors such as SrGa2S4:Ce, CaGa2S4:Ce, and BaAl2S4:Eu were developed as described in JP-A 7-122364, JP-A 8-134440, Shinshu Univ. Technical Report, EID 98-113, pp. 19-24, and Jpn. J. Appl. Phys., Vol. 38 (1999), pp. L1291-1292. From the BaAl2S4:Eu phosphor, light of CIE1931 chromaticity coordinates (0.12, 0.10) is obtained. However, there is a need for a blue phosphor having a higher color purity. If blue light having a higher purity, especially blue light having CIE1931 chromaticity coordinates of x less than about 0.2 and y less than 0.10, that is, NTSC blue of (0.14, 0.08) is developed, then a display of better quality becomes available.
An object of the invention is to provide a phosphor thin film eliminating a need for filters, having a satisfactory color purity and useful as the blue color in full-color EL displays, a method for preparing the same, and an EL panel.
This and other objects are attained by the present invention which is defined below as (1) to (6).
(1) A phosphor thin film formed of a matrix material comprising barium thioaluminate as a main component and having europium added thereto as a luminescent center, said matrix material further containing magnesium.
(2) The phosphor thin film of (1) wherein magnesium is added in such an amount as to give an atomic ratio Mg/(Ba+Mg) between 0.05 and 0.8.
(3) The phosphor thin film of (1) or (2) which emits blue light having chromaticity coordinates of x less than 0.2 and y less than 0.10.
(4) An EL panel having the phosphor thin film of any one of (1) to (3).
(5) A method for preparing the phosphor thin film of any one of (1) to (3) by an evaporation process, placing at least an aluminum sulfide source, a barium sulfide source having a luminescent center added thereto, and a magnesium metal source in a vacuum chamber, and evaporating aluminum sulfide, barium sulfide and magnesium metal from the respective sources and depositing the evaporated substances on a substrate while binding the substances together, thereby forming a sulfide phosphor thin film.
(6) A method for preparing the phosphor thin film of any one of (1) to (3) by an evaporation process,
placing at least a barium thioaluminate source having a luminescent center added thereto and a magnesium metal source in a vacuum chamber, and
evaporating barium thioaluminate and magnesium metal from the respective sources and depositing the evaporated substances on a substrate while binding the substances together, thereby forming a sulfide phosphor thin film.